1. Field of the Invention
The present invention relates to multi-crystalline silicon, which is useful as, in particular, a material of solar cells, a multi-crystalline silicon wafer, a method for producing the crystal, and a multi-crystalline silicon solar cell using it.
2. Related Art
Characteristics of solar cells are explained first with reference to the materials for substrates constituting the solar cells. When solar cells are classified based on the materials of-substrates, they are categorized into three groups, "silicon crystal solar cells", "amorphous silicon solar cells", and "compound semiconductor solar cells". Further, the silicon crystal solar cells include "single crystal silicon solar cells" and "multi-crystalline silicon solar cells". Among these, solar cells exhibiting high conversion efficiency, which is the most important characteristic as a solar cell, are the "compound semiconductor solar cells", and their conversion efficiency reaches almost 25%. However, production of compound semiconductors, the materials of the compound semiconductor solar cells, is extremely difficult, and thus they have a problem for wide use in view of their production cost. Therefore, their applications have been limited.
The term "conversion efficiency" used herein is a value indicating "a ratio of energy which can be drawn as electric energy converted from light energy by a solar cell relative to energy of light entering the solar cell", and more specifically, it refers to a value defined by the following equation: [Conversion efficiency]=[Electric power which can be drawn out from unit area of cell]/[light energy irradiated on the unit area of the cell].times.100 (%)
As solar cells exhibiting high conversion efficiency in the next place to the compound semiconductor solar cells, there can be mentioned single crystal silicon solar cells, and they show generation efficiency of around 20%, which means that they show conversion efficiency near that of the compound semiconductor solar cells. However, the material cost of such single crystal silicon solar cells constitutes about 1/2 of their whole production cost, and thus they also has a drawback of difficulty in cost reduction.
Therefore, because of low production cost of solar cell substrates, multi-crystalline silicon solar cells have been put into practical use and most frequently produced at present, in spite of their conversion efficiency of 5-15%, which is inferior to those of the aforementioned two kinds of solar cells.
Now, the method for producing usual multi-crystalline silicon solar cells will be explained briefly. First, in order to obtain silicon wafers used as substrates of solar cells, electronic grade-silicon is charged into a crucible made of quartz or the like, then the silicon in the crucible is melted by heating the crucible in a heating region, and multi-crystalline silicon is grown by cooling the crucible by descending the crucible from the heating region to obtain a multi-crystalline silicon ingot. Further, this ingot is sliced into thin wafers having a thickness of, for example, about 300 .mu.m, and mechanical damages on the wafer surfaces is removed by etching the wafer surfaces with a chemical solution to obtain wafers (substrates) to be used it solar cells. Each of these wafers is subjected to a diffusion treatment for diffusing impurities (dopant) to form a PN junction on one side of the wafer, then an anti-reflection film is provided for reducing loss of light energy due to light reflection on the surface on which solar light is irradiated, and finally electrodes are attached to the both sides of the wafer to complete a solar cell.
While the demand of solar cells has been increasing in recent years as one of clean energy sources with the background of the problem of environmental protection, their higher energy cost compared with usual commercial electric power causes obstruction of their wide use. For the cost reduction of silicon crystal solar cells, it is important to reduce the production cost of substrates, and it can be said that multi-crystalline silicon solar cells of which substrates can be produced at low cost satisfy such demand.
On the other hand, however, it is also important to further increase the conversion efficiency of solar cells. Solar cells generate electromotive force through separation of carriers generated by light by an internal electric field. Therefore, it is desirable that lifetime of the generated carriers is as long as possible, and longer carrier lifetime affords higher conversion efficiency.
However, multi-crystalline silicon produced by melting electronic grade-silicon and solidifying it by cooling, as is the current mainstream of the production method of multi-crystal ingots, suffers from a problem concerning conversion efficiency, when solar cells are produced from it. That is, in such solar cells, if a certain period of time passes after irradiation of strong light, lifetime of carriers is reduced in substrates of the solar cells. Therefore, conversion efficiency is reduced, and sufficient conversion efficiency cannot be stably obtained. Therefore, improvement of this problem is desired from the viewpoint of performance of solar cells.
It is known that the cause of the reduction of carrier lifetime caused by irradiation of strong light, when solar cells are produced from such multi-crystalline silicon, is influence of boron and oxygen that exist in the multi-crystal substrates. The conduction type of wafers currently used in solar cells is mainly P-type, and P-type wafers are usually added with boron as a dopant. Further, oxygen also exists in crystals of the multi-crystalline silicon. For these reasons, the multi-crystalline silicon suffers from the problem that lifetime characteristics are affected by boron and oxygen in P-type multi-crystalline silicon, and there is caused photodegradation.
On the other hand, it is not preferable to significantly reduce the doping amount of boron, since wafers having a resistivity as low as possible are desirable as wafers for solar cells. Further, as also for oxygen, it is also inevitably incorporated into multi-crystalline silicon, because quartz is used in its production, and silicon melt is contained in it in order to increase purity of multi-crystalline silicon. In addition, if the oxygen concentration in crystal silicon is unduly lowered, mechanical strength of the crystal silicon would be degraded. Therefore, conventional solar cells obtained by using multi-crystalline silicon suffer from problems of reduction and instability of conversion efficiency, when they are used for a long period of time.